1. Field of the Invention
The present invention relates to a fixing device and an image forming device equipped with the fixing device.
2. Description of the Related Art
A fixing device for fixing toner images on paper by heating and pressure application is equipped with a heater having a heat source and a pressurizer disposed so as to make contact with the heater. Fixing is performed for paper by passing the paper through a nip section formed between the heater and the pressurizer. In a heating roller type fixing device, a heating roller is used as the heater, and a pressure roller is used as the pressurizer. The heating roller is heated by the heat source, and the pressure roller is heated by the heating roller.
During the fixing, when paper passes through the nip section, heat is taken from the heating roller and the pressure roller by the paper. The heating roller is made of metal and formed into a cylinder and has a relatively small heat capacity. In addition, the surface temperature of the heating roller is controlled to a predetermined set temperature. Therefore, the problem of the temperature decrease occurred in the heating roller is resolved immediately. On the other hand, the pressure roller is made of rubber and formed into a cylinder and has a relatively large heat capacity. Therefore, the problem of the temperature decrease occurred in the pressure roller is not resolved for a while.
If the temperature of the pressure roller is too low, improper fixing occurs due to insufficient temperature. On the other hand, if the temperature of the pressure roller is too high, offset (paper sticking to the heating roller) or curling (paper bending) occurs due to excessive temperature. For this reason, for the purpose of properly performing fixing for paper, it is necessary to determine the set temperature in consideration of the amount of temperature decrease in the pressure roller. In the following descriptions, the set temperature required to properly perform fixing for paper is referred to as a required temperature. The required temperature is set higher as the amount of temperature decrease is larger so that the amount of temperature decrease is compensated.
The amount of temperature decrease differs depending on a kind of paper. The kinds of paper herein indicate the differences in the thickness (basis weight) and the size of paper. The basis weight is a weight per unit area of paper. Therefore, the required temperature is specified depending on the kind of paper. The required temperature required for A4 size regular paper differs from the required temperature required for A3 size thick paper.
In the case that the kind of paper is changed, the set temperature is required to be changed. The user of an image forming device specifies the kind (basis weight and size) of paper using the operation panel (operation unit) or the like in the image forming device. The fixing device (the image forming device) sets the set temperature on the basis of the specified kind of paper so that the set temperature becomes the required temperature corresponding to the kind of paper.
Japanese Patent Application Laid-open Publication No. 6-289750 (Patent document 1) describes a fixing device equipped with a function of automatically changing the set temperature. The fixing device described in Patent document 1 is a film heating device in which film is used as a heater. In Patent document 1, the set temperature of the film is changed automatically by detecting the temperature decrease speed (temperature change per unit time) after the end of printing. The temperature decrease speed (the degree of ease to cool) of the film changes depending on the temperature environment of the fixing device. In other words, the set temperature is changed depending on the degree of ease to cool the film.
In the above-mentioned conventional fixing device, the set temperature is set on the basis of the kind of paper specified using the operation panel. Therefore, in the case that the user has specified an incorrect kind, that is, in the case that the specified kind differs from the kind of paper actually set inside the image forming device, the set temperature becomes different from the required temperature of the actual paper. As a result, improperly printed paper is generated due to insufficient or excessive temperature. The printed paper is herein defined as paper on which printing is performed.